With the rapid development of lightwave communications, low cost, high performance optical receivers are needed for a variety of system applications. It is anticipated that the monolithic integration of a photodetector with a low noise preamplifier offers the potential advantages of reduced parasitic capacitances and inductances, as well as, improved device performance, higher reliability and lower manufacturing cost. Furthermore, since silica fibers have a lower attenuation in the long wavelength range (1.3-1.6 .mu.m), a high degree of interest has been generated by integrated optical receivers which operate in this spectral range.
Unfortunately, the integration of receiver components operating in the long wavelength range necessitates the utilization of Indium Gallium Arsenide (InGaAs) p-i-n photodetectors and, hence, a transistor technology based on Indium Phosphide (InP) substrates in order to avoid strained layer epitaxy because of lattice mismatch. Unlike Silicon (Si) and Gallium Arsenide (GaAs), however, InP does not have a well-established transistor technology. Among the first InGaAs integrated photoreceivers were those utilizing a p-i-n photodiode and amplifier made from field effect transistors (FETs). Typically, a p-i-n photodiode requires a few micrometer thick, lightly doped epitaxial photoabsorbing layer on a heavily doped substrate in order to achieve high speed and high responsivity, while an FET requires a few hundred Angstrom thick, heavily doped channel on a semi-insulating substrate in order to achieve high transconductance. Due to this conflicting characteristic in the epilayer structure for the photodiode and the FET, either ion-implantation or diffusion into the InP substrate is required, which disadvantageously increases the processing complexity. Examples of integrated photoreceivers based on the above p-i-n/FET approach are shown in Kim et al., IEEE Electron Device Letters, Vol. 9, No. 9 pp. 447-9 (1988) and Renaud et al., Journal of Lightwave Technology, Vol. 6, No. 10, pp. 1507-11 (1988).
Other alternatives to the above p-i-n/FET approach having better materials compatibility have recently been reported in the technical literature. These alternatives utilize either metal-insulator semiconductor FETs(MISFETs) or high electron mobility transistors (HEMTs) instead of conventional FETs for fabricating the amplifier. See, for example, Antreasyan et al., IEEE Photonics Technology Letters, Vol. 1, No. 6 pp. 123-5 (1989) and Nobuhara et al., Electron Letters, Vol. 24, pp. 1246-48 (1988). Typically, in the above alternatives, a two growth epilayer process is required so that the separate layers for the photodetector and transistor are fabricated in different growth runs. Furthermore, InP MISFETs are still substantially prone to drain current drift phenomena, which may prevent long term functionality, while the speed of HEMTs is critically predicated on device dimensions.
An integrated photoreceiver using an InP/InGaAs heterostructure phototransistor (HPT) has recently been developed by one of the inventors as an alternative to the p-i-n/FET approach in order to achieve greater materials compatibility and, moreover, to overcome some of the limitations imposed by the utilization of MISFETs and HEMTs. See, Chandrasekhar et al., Electronic Letters Vol. 24, No. 23 pp. 1443-4 (1988). However, the dual functionality of the HPT, that is as photodetector and amplifier, results in a compromise in the optimal performance of the photonic and electronic function thereof.
One solution to the control optimization has been the monolithic integration of an HBT, HEMT and p-i-n photodiode on a patterned InP substrate. See, Sasaki et al., IEDM 89, pp. 896-8. HBTs, which incorporate an emitter region with a bandgap energy greater than that of the base region, allow a higher base doping and lower emitter doping that result in excellent device performance. For a discussion on HBTs, see U.S. Pat. No. 4,794,449, which is incorporated herein by reference. In fact, when compared with semiconductor FETs, HBTs offer higher transconductance and drive capability. Accordingly, the utilization of HBTs results in a high sensitivity monolithic photoreceiver. More importantly, HBTs may be fabricated with modest lithographic design rules while still maintaining speed performance and noise margin characteristics comparable with FETS.
The integration shown by Sasaki et al. was accomplished using the selective regrowth capabilities of metal-organic chemical vapor deposition (MOCVD). While the epitaxial layers used to form the HEMT and HBT were stacked on each other for ease of fabrication, in order to achieve planarity for gate lithography, areas on the substrate where the HBT and HEMT were to be realized had to be etched to form trenchs, i.e., a pattering of the substrate. Thus, by requiring the substrate to be initially patterned, a complex two step MOCVD process had to be incorporated in order to subsequently integrate the HBT and photodiode.
In view of reducing the processing complexity, it is therefore desirable to develop a monolithic integrated HBT photoreceiver not only having enhanced materials compatibility but also separate optimization control over the photonics and electronics functions.